EUV lithography (EUVL) is the one of the candidates for Next Generation Lithography (NGL) for microchip fabrication. EUVL is designed for printing those microchips whose minimum feature size is 30 nm or even smaller. To achieve such a small feature size, EUVL utilizes a shorter wavelength of radiation. Like other lithographic systems, EUVL consists of a light source, mask, demagnifying optics, and photoresist on the wafer. To utilize such short wavelengths of radiation in the optical system, no lenses can be used, since most materials absorb these wavelengths. Mirrors must be used throughout. The mirrors for this application are very specialized, consisting of two different materials stacked together in very thin bilayers, known as Multi-layer mirrors (MLMs) or Bragg reflectors. EUVL mirrors are silicon-molybdenum MLMs whose reflectivity peaks at about 13.5 nm. The surface quality and uniformity of these optics must be extremely good due to the short wavelength.
EUVL also requires sufficient light source power and lifetime to reach the required throughput for production. The power requirement, which is determined by the lithographic industry, is being specified in terms of EUV power at the intermediate focus (IF) of the optical system, which is the border between the light source segment and scanning (also called “stepper”) segment in the lithographic system. The specified EUV power is 115 W, within 2% bandwidth of 13.5 nm. Also, the lifetime of the light source is required to be 30,000 hours as described in Presented at International Sematech Source workshop, February 2004, Santa Clara, Calif. [1].
The radiation for EUVL is usually generated from a high-temperature plasma. There are two methods of plasma production currently being investigated in this research field: laser plasma (LP) and gas discharge plasma (GDP). Both techniques are capable of creating such high-temperature plasmas. An LP light source consists of a high-power laser, target delivery system, and a vacuum chamber in which the plasma is created. Similarly, GDP consists of a high-power current source, electrodes, and a vacuum chamber. One advantage of LP over GDP is that the material surrounding the plasma is further away in an LP system, which allows a higher repetition rate of the plasma generation in order to reach the EUV light source power requirement. There is also an advantage of LP in terms of collecting useful radiation from the plasma. Due to the extensive electrode structure necessary for GDP, only a part of the radiation can be collected. In other words, the collection solid angle is limited by the electrodes.
LP is produced by a material which is under strong laser irradiation, usually with intensities of more than 1010 W/cm2. A single focusing lens with focal length of a few centimeters can be used for obtaining such a high-intensity laser focus. At these intensities, all of the material within the focus region of the laser beam will be ionized and become a high-temperature plasma. With a suitable material like tin, xenon, or lithium inside the target material, the plasma produces EUV emissions at 13.5 nm.
Many approaches are being investigated for EUVL light sources, such as xenon cluster jet which is described in U.S. Pat. No. 6,324,256; xenon filament described in U.S. Pat. No. 6,002,744), tin solid planar target, and others. Droplet target technology is one of the most promising for EUV lithography light sources. The advantage of this target over solid targets or other geometries is the reduced but sufficient mass of the target for EUV radiation. It is then possible to eliminate excess material from the plasma, which would tend to damage the MLM surfaces surrounding the plasma source. This scheme is necessary to achieve the EUVL light source lifetime requirement which is determined by the reflectivity lifetime of the mirror. The mirror reflectivity drops when the number of layers is decreased by ablation of the mirror surface, caused by target material emitted from the plasma. There are several approaches to generate appropriate droplet targets: water-based solution with tin doping as described in M. Richardson, et al. Journal of Vacuum Science and Technology B, volume 22, number 2, pp 785, (2004) and U.S. Pat. No. 6,862,339, liquid xenon described in U.S. Pat. No. 6,855,943, and liquid lithium described in Presented by Cymer Corp. at 3rd EUVL symposium, November 2004, Miyazaki, Japan.
The targets have to be positioned correctly within the laser focus region to produce a suitable-temperature plasma for EUV radiation. The size of the target ranges from a few tens of microns to possibly a few hundreds of microns where the laser focus region is adjusted to about the same size as the target. The target droplets are generated at high repetition rates, from a few kilohertz to a few tens of kilohertz, and travel at a few tens of meters per second. The laser pulse, with duration of a few nanoseconds to a few tens of nanoseconds, is focused. Any slight displacement of the target within the laser focal region during the laser pulse duration can cause reduced EUV emission. Thus high-precision controls over target positioning are required, both spatially within the laser focal region and temporally within the laser pulse duration.
An open-loop system is a simple approach for controlling the target positioning and laser pulse timing; i.e., a simple synchronization of the electrical signals controlling the droplet targets and laser pulse can be used. However, the generation of the droplet targets depends on many physical parameters of the target material and the orifice of the target supply. For instance, when the temperature of the target material changes, the viscosity of the material changes which leads to a slight change in the velocity of the droplet. Then the synchronization of the target and laser pulse has to be adjusted again. Similarly the physical profile of the orifice changes when the target material is deposited on the orifice over time. This changes the trajectory and stability of the targets. Again the target positioning has to be adjusted. These changes usually occur slowly, especially in long-term operation. In addition, EUVL requires full-time operation to meet the industry's fabrication throughput requirements. Therefore a closed-loop system is necessary to keep the target positioned correctly for best operation.
Several approaches have been published for controlling target positioning and synchronizing the target and the laser pulse. One approach is to generate the target dispensing signal from generated target signals as described in O. Hemberg, B. A. M. Hansson, M. Berglund, and H. M. Hertz, Journal of Applied Physics, Vol. 88, pp. 5421-5424 (2000) and in J-Q. Lin, et al. Proceedings of the SPIE, Volume 5374, pp. 906-911 (2004). However, both approaches are only efficient in one dimension. When droplet trajectory changes by a certain amount the probe laser light no longer produces shadows, making it impossible to trigger the laser pulse. Another approach is to steer the droplet spatially as described in U.S. Pat. No. 6,792,076. With a steering actuator, any trajectory change can be compensated, limited by the size and velocity of the actuator. However, the temporal adjustment has to be done separately when the velocity of the droplets changes.